Proofing is the procedure used by the printing industry for creating representative images that replicate the appearance of printed images without the cost and time required to actually set up a high-speed, high-volume printing press to print an example of the images intended. Ideally, these representative images, or proofs, are generated, if in color, from the same color-separations used to produce the individual color printing plates used in printing presses so that variations in the resulting images can be minimized. Various proofing systems have been devised to create the proofs and have included the use of smaller, slower presses as well as means other than presses, such as photographic, electrophotographic, light imaging and nonphotographic processes.
The proofs generated are judged for composition, screening, resolution, color, editing and other visual content. The closer the proof replicates the final image produced on the printing press, as well as the consistency from image to image, from press to press and shop to shop, the better the acceptance of the proofing system by the printing industry. Other considerations used in judging proofing systems include reproducability, cost of the system as well as cost of the individual proofs, speed and freedom from environmental problems.
It is generally necessary to produce a "hard copy" (i.e. an image actually formed on a sheet of paper or other proof receiving material) before it can be fully assessed for final printing approval. Thus, each of the proofing systems require the use of some form of output device or printer which can produce a hard copy, on the type of media to be used in the printing process, so that the image can be properly evaluated. It is to the field of proofing output devices that the present invention is directed.
While purely photographic processes can provide accurate reproductions of images, they do not always replicate the reproduction resulting from printing presses nor is the media on which the image is to be printed necessarily the same media as used in the photographic process. Since the appearance of the final printed image is affected by the characteristics of the media upon which it is to be printed, the chemical make-up of the substance used to produce the image or other materials used to produce the hard copy are critical. Thus the ability to form the proof image on the material actually used in the printing process is a determining factor in the selection of the proofing system.
One form of image representation is produced on a plastic receiver film in which the plastic receiver is passed through a color copier and while said plastic receiver is supported on a rotatable copy drum, dye is transferred by sublimation to the plastic receiver to form a full color image on said receiver.
The problem, however, is how to transfer this full color image to the paper or other media that is to be used in the printing process without running or migrating of the dyes that make up the image, so that the image can be evaluated on the paper or media that is to be used in the printing process.